This invention relates to making thin strip and more particularly casting of thin strip by a twin roll caster.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotating horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces. The solidified metal shells are brought together at the nip between the casting rolls to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel, such as a tundish or distributor, from which it flows through to a metal delivery nozzle located above the nip, which directs the molten metal outwardly below the surface of a casting pool supported on the casting surfaces of the rolls above the nip. This casting pool is typically confined at the ends of the casting rolls by side plates or dams held in sliding engagement adjacent the ends of the casting rolls.
In casting thin strip by twin roll casting, the metal delivery nozzles typically receive molten metal from a movable tundish and deposit the molten metal in the casting pool in a desired flow pattern. Previously, various designs have been proposed for delivery nozzles involving a lower portion submerged in the casting pool during a casting campaign, and having side openings through which the molten metal is capable of flowing laterally into the casting pool outwardly toward the casting surfaces of the rolls. Examples of such metal delivery nozzles are disclosed in Japanese Patent No. 09-103855 and U.S. Pat. No. 6,012,508.
In the past, the formation of pieces of solid metal known as “skulls” in the casting pool in the vicinity of the confining side plates or dams have been observed. The rate of heat loss from the casting pool is higher near the side dams (called the “triple point region”) due to conductive heat transfer through the side dams to the casting roll ends. This localized heat loss near the side dams has a tendency to form “skulls” of solid metal in that region, which can grow to a considerable size and fall between the casting rolls and causing defects in the cast strip. An increased flow of molten metal to these “triple point” regions, the regions near the side dams, have been provided by separate direct flows of molten metal to these triple point regions. Examples of such proposals may be seen in U.S. Pat. No. 4,694,887 and in U.S. Pat. No. 5,221,511. Increased heat input to these triple point regions has inhibited formation of skulls.
Moreover, Australian Patent Application 60773/96 discloses a method and apparatus in which molten metal is delivered to the delivery nozzle in a trough closed at the bottom. Side openings are provided through which the molten metal flows laterally from the delivery nozzle into a casting pool in the vicinity of the casting pool surface. However, in such metal delivery nozzles, there has been a tendency to produce thin cast strip that contains defects known as ridges. Further, there has been concern for extending the useful life of the delivery nozzles and in turn reducing the cost of producing thin cast strip. Specifically, there remained concern for wear on the delivery nozzle caused by the impact of the molten metal due to ferrostatic pressure, and turbulence caused as the molten metal moved through the delivery nozzle to discharge laterally into the casting pool below the meniscus of the casting pool.
The present invention provides an apparatus and method for continuous thin strip casting that is capable of substantially reducing and inhibiting such defects such as ridges in the cast strip, and at the same time reducing wear in the delivery nozzles and costs in thin strip casting. By testing, we have found that a major cause of such strip defects is thinning of the shells during casting caused by localized washing of solidified shells during formation from over flow of the molten metal into the casting pool. We have found by changing the delivery nozzle that the flow of molten metal to an upward flow into the casting pool that there is less potential to cause thinning of the solidified metal shell during formation. This improved flow from the delivery nozzle into the casting pool is particularly notable in the region where the casting pool meets the casting surfaces of the rolls, generally known as the “meniscus” or “meniscus regions” of the casting pool.
Disclosed is method of casting metal strip comprising:                (a) assembling a pair of casting rolls laterally disposed forming a nip between them,        (b) assembling an elongated metal delivery nozzle extending along and above the nip between the casting rolls, with at least one segment having a main portion with outlets adapted to upwardly discharge a flow of molten metal into a casting pool along opposite sides of the segment,        (c) introducing molten metal through the elongated metal delivery nozzle to form a casting pool of molten metal supported on the casting rolls above the nip, such that molten metal flows from the segment through the outlets adapted to discharge the flow at an upward angle into the casting pool, and        (d) counter rotating the casting rolls to form shells on the casting rolls and bring the shells together at the nip to deliver cast strip downwardly from the nip.        
Also disclosed is a metal delivery apparatus for casting metal strip comprising at least one elongated segment having a main portion and an inner trough extending longitudinally through the main portion with end walls at opposite ends thereof, the inner trough communicating with outlets along opposite sides of each segment adapted to upwardly discharge a flow of molten metal into a casting pol.
The outlets in the method of casting metal strip and of the metal delivery apparatus may have an upward directional discharge angle between 15 degrees and 45 degrees or between 20 degrees and 30 degrees from horizontal. Also, the outlets in the method of casting metal strip and of the metal delivery apparatus may have a discharge with a lateral spread angle between 0 degrees and 30 degrees or between 5 degrees and 15 degrees.
The outlets of the metal delivery apparatus may be offset along opposite sides of the segment and may overlap in longitudinal position. This offset and overlap of the outlets on opposite sides of the segment of the metal delivery nozzle provided further potential for lessening of thinning of the metal shells during formation on the casting rolls and produce less defects in the cast strip.
The at least one segment may have an inner trough extending longitudinally through the main portion with end walls at opposite ends thereof, the inner trough communicating with outlets along opposite sides of each segment.
The outlets may extend to adjacent the end of each segment and may have an end portion with the inner trough extending into the end portion, the end portion having a reservoir portion having passages adapted to deliver molten metal to a casting pool near side dams. This increased flow of molten metal to these “triple point” regions, the regions near the side dams, have been provided by separate direct flows of molten metal to these triple point regions and inhibits formation of “skulls” in the casting pool.
Various aspects of the invention will be apparent from the following detailed description, drawings, and claims.